Diaphragms of the above kind in illumination systems for microlithographic projection exposure systems are known, for example, from U.S. Pat. No. 5,473,408. In the illumination system shown here for a microlithographic projection exposure system, a reflecting diaphragm having a circularly-shaped diaphragm opening is mounted directly ahead of a rod integrator. On the one hand, light can enter into the rod integrator via the diaphragm opening and, on the other hand, light, which is reflected back from the reticle and again arrives in the rod integrator, is reflected an additional time at the reflecting diaphragm surface and is again supplied to the useful light. The diaphragm opening has a diameter which is significantly less than the height of the rod.
U.S. Pat. No. 5,601,733 likewise discloses diaphragms which are mounted forward of a rod integrator. As shown in U.S. Pat. No. 5,473,408, the diaphragms are provided with a reflecting surface on the side, which faces toward the rod integrator, in order to again supply to the useful light the light which is reflected back into the rod integrator from the illumination system. A laser having a low “etendue” (phase space volume) is used as a light source. For this reason, the light can be focused with the condenser optics onto the diaphragm so that the light passes through the diaphragm opening without vignetting and the diaphragm opening has a diameter as small as possible.
Illuminating systems for microlithography with rod integrators are also disclosed in U.S. Pat. No. 5,675,401. A mercury high pressure lamp is used here as a light source. In addition to a virtual spherically-shaped emission characteristic, such light sources have a finite expansion so that they have a significantly greater etendue compared to laser light sources. In U.S. Pat. No. 5,675,401, a condenser optic focuses the light onto the entry surface of the rod integrator and generates a light spot there. This light spot is round as long as the light source has an expansion, which is rotationally symmetrical to the optical axis, and the optical components in the condenser optic have rotationally symmetrical optical effects. Precisely in wafer scanners, rod integrators are used whose entry surface has a high aspect ratio of rod width to rod height, for example, an aspect ratio of 2:1 or greater. Here, it can happen that the expansion of the light spot is greater than the height of the rod. This leads to the situation that the light is vignetted. As a rule, however, the ray angles, which occur at the entry surface, are dependent upon the spacing from the optical axis. For this reason, the vignetting leads to a so-called elliptical pupil illumination. An elliptical pupil illumination is understood to mean the intensity distributions in the pupil planes which, in the quadrants arranged about a horizontal axis, have a greater total intensity than in the quadrants arranged about a vertical axis and therefore lead to a different resolution capacity for horizontal and vertical structures in the imaging of horizontal and vertical structures with projection objectives used in microlithography.